Some time back I wrote up a comparison of the Road Morph G and the similar Lezyne pump. These are the most popular for touring.
Comparing Topeak Road Morph G and Lezyne Micro Floor Drive Pumps.
Without braze-ons, the best way to carry it may be in a pannier. I have carried either a Lezyne or Road Morph G on every one of my bike tours.
And if you are not carrying a pannier, an older style frame pump may work best. I have some older frame pumps and carry one of those on my road bike and sometimes on my rando bike. These pumps don't have gauges, often you just pump it up as much as you can and then use a better pump later when you have access to one.
In this photo, I have an old frame fit Zefal pump parallel to and below my top tube. That bike does not have a pump peg on the head tube, but I use a velcro strap near the front of the bike that keeps the pump from slipping down the headtube. I am not saying this pump is the best option, just saying it is one of many options.
If you do not have a water bottle cage on your seat tube, a pump like that easily fits parallel to and in front of the seat tube, that was the most common way to carry these pumps decades ago before people started putting bottle cages on seat tubes.
With frame pumps, the pump length is specific to a frame, the pumps are sold in different lengths to fit different size bikes.
Your question on CO2, I can't answer that because I have never used CO2.